Stereologic techniques will be used to quantitatively describe normal atrial and ventricular myocardial cells in the finch, mouse, chicken and cat as an anatomical basis for comparing functional parameters such as calcium compartmentalization during the excitation-contraction cycle and as baseline information for the study of abnormal cardiac states such as hypertrophy. Hearts perfused with fixative with and without, lanthanum and horseradish peroxidase will be examined by light and electron microscopy. Isolated cells will also be evaluated. The structures to be investigated include the sarcolemma, mitochondria, transverse tubular system and the components of the sarcoplasmic reticulum (free, junctional, and extended junctional SR) volume fractions, surface area per unit volume of myocardial cells, and number per unit volume will be determined. These data will be compared with functional data such as heart rate, and calcium kinetics of isolated sarcoplasmic reticulum.